1. Field of the Invention
The present invention relates to a high frequency module used in a communications apparatus for a wireless local area network (LAN), for example.
2. Description of the Related Art
Attention has been recently drawn to a wireless LAN that forms a LAN through the use of radio waves as a technique for constructing a network easily. A plurality of standards are provided for the wireless LAN, such as the IEEE 802.11b that uses a 2.4 GHz band as a frequency band and the IEEE 802.11a and the IEEE 802.11g that use a 5 GHz band as a frequency band. It is therefore required that communications apparatuses used for the wireless LAN conform to a plurality of standards.
Furthermore, the communications status on the wireless LAN varies depending on the location of the communications apparatus and the environment. It is therefore desirable to adopt a diversity for choosing one of a plurality of antennas whose communications status is best.
In the communications apparatus for the wireless LAN, a circuit portion (hereinafter called a high frequency circuit section) that is connected to antennas and processes high frequency signals is incorporated in a card-shaped adapter, for example. In addition, it is expected that the communications apparatus for the wireless LAN is installed in a mobile communications device such as a cellular phone. A reduction in size of the high frequency circuit section is therefore desired.
A type of mobile communications device such as a cellular phone is known, wherein a high frequency circuit section is formed as a module operable in a plurality of frequency bands. For example, Japanese Published Patent Application (hereinafter referred to as ‘JP-A’) 2003-152588 discloses a module incorporating two diplexers and a single switch circuit. In this module, the switch circuit switches one of the two diplexers to be connected to the single antenna. Each of the diplexers separates two signals in different frequency bands from each other.
JP-A 2000-349581 discloses a typical diplexer comprising a combination of a low-pass filter and a high-pass filter.
JP-A 11-55156 discloses a switch circuit incorporating a GaAs field-effect transistor. In this switch circuit, a transmission section and a reception section are connected to terminals through which transmission signals and reception signals pass, respectively, through capacitors for blocking direct currents.
Each of JP-A 2001-136045 and JP-A 2001-119209 discloses a module incorporating a duplexer for separating transmission signals from reception signals. This module incorporates two band-pass filters each of which is made up of inductor conductors and capacitor conductors of a layered structure made up of a plurality of insulating layers, a plurality of inductor conductors and a plurality of capacitor conductors. Furthermore, these two publications disclose a technique in which the axis of the inductor conductors making up one of the band-pass filters is made orthogonal to the axis of the inductor conductors making up the other of the band-pass filters. In addition, the two publications disclose a diplexer as another example of the module to which the above-mentioned technique is applied. These publications disclose that the diplexer is formed by combining a low-pass filter and a high-pass filter, for example.
As previously described, it is desirable that a communications apparatus for the wireless LAN conform to a plurality of standards whose operable frequency bands are different. It is therefore desired that the high frequency circuit section of the communications apparatus for the wireless LAN be capable of processing transmission signals and reception signals in a plurality of frequency bands. To achieve this, a configuration is conceivable in which the high frequency circuit section comprises a plurality of diplexers for separating signals in two different frequency bands from each other, and a switch circuit for connecting one of the diplexers to an antenna terminal. In this case, a direct-current control signal for controlling switching of the state is applied to the switch circuit. It is necessary to prevent a direct current resulting from the control signal from flowing into the diplexers. To do so, a technique is conceivable in which circuits for processing transmission signals and reception signals respectively are connected through capacitors for blocking direct currents to terminals through which transmission signals and reception signals pass respectively, as disclosed in JP-A 11-55156.
Consideration will now be given to a configuration in which a diplexer as the circuits for processing transmission signals and reception signals is connected through a capacitor for blocking direct currents to the terminals through which transmission signals and reception signals pass respectively. This configuration has a problem that will now be described. A diplexer is a device for separating signals in two different frequency bands from each other as previously mentioned. If a capacitor for blocking direct currents is provided between the switch circuit and the diplexer, the capacitor has an influence on passing characteristics along two signal paths corresponding to the two frequency bands in the diplexer. The problem is that it is difficult to predetermine the capacitance of the capacitor for blocking direct currents so that the passing characteristics along the two signal paths are both favorable. That is, if the capacitance of the capacitor is predetermined so that the passing characteristic along one of the paths is favorable, the passing characteristic along the other of the paths deteriorates. If the capacitance of the capacitor is predetermined so that the passing characteristics along the two paths are balanced, the passing characteristics both deteriorate to some extent.